Wait, I'd better clarify and admit to something here.
I asked my children NOT to comment on FB on anything their father said about OW. I told them that I didn't feel it was right that he posted there about someone else while we are still married and no divorce is filed, that it isn't right that he is seeing someone else while we are still married and that I would not condone his actions nor would I take the blame for them.
Therefore, my explanation to them was that I felt that to be fair to both of us and not be in the middle, that I would prefer that they not comment to their father or me on FB about this situation, instead they should talk to us one on one and make their decision based on what they feel is right. Just not in a public forum because, regardless, we are both their parents and they are stuck with both of us for life.